Many laboratory and factory operations require removal of substantially all suspended particles from the air in a work area, in order to prevent various kinds of contamination. Standard filter units have been developed for this purpose, as well as plenum structures supporting them and providing the necessary pressure differential to induce the flow of air through the filters and into the room. These plenum structures are often modular; and can occupy the entire ceiling area, or be installed in an opening in the ceiling. In many instances, space above the modules is at a premium, and special module design must be used to minimize the vertical height, as well as the need for clearance space above it. The principal problem here is in providing a uniform distribution of low-velocity air at the increased pressure over the filter units. Height restriction eliminates many of the more obvious arrangements for converting the high-velocity blower exhaust to the low-velocity, high-pressure, flow over the filters. Such conversion must be done without such sophisticated arrangements that would cause the cost of the modules to get out of hand. This requires that simple sheet-metal formation techniques be preserved, and standard off-day-shelf blowers be utilized. The present invention has accomplished this, as a result of extensive experimentation directed at getting the most filter pressure out of a given motorized blower within the design limitations imposed by height restrictions.